I'm basically wanting to build a signal meter detector, but with less of the meter bit. Just a detector, that illuminates an LED and sounds a buzzer if a signal is present.

I already a DVB signal meter, however, as I need to construct a circuit for my coursework, would like to make the detector.

The intention of it is, to position a TV aerial, then connect the aerial to the detector, to see if a signal is present or not - if that makes sense - so when the aerial is pointing in the right direction, the buzzer sounds and LED illuminates. As a bonus, I guess this could also be used for testing for a signal through a cable - but the main intention is as described above.

A signal?
You do not say where you are.
In my part of Canada there are about 50 AM radio stations, 50 FM radio stations, 30 TV stations plus police, fire dept., ambulance and hundreds of microwave signals.
A simple detector will detect most of them all at the same time.

In my city there is a communications tower where most signals come from so turning an aerial is not necessary.

You need a complicated radio circuit with many tuned circuits to select only one of them.

One signal is extremely weak which is why a radio circuit has many amplifying stages.
Then you must design a circuit that will illuminate an LED and sound a buzzer if a TUNED AND AMPLIFIED signal is present.

I'm in the UK. Okay I see what you mean. Maybe then, if I could create a detector, that when I inject a signal into the cable, it detects it at the other end? I could relate this to a scenario as I have lots of aerial cables all close together and I do not know which ones go where, so this could be useful. So, in theory then, I need to create two devices? One that will inject the signal into the cable, and another which will detect it?

I've used it, but it also traces through 1 splitter. 2 splitters for only sync light and no tone, 3 splitters and no signal (-6dB or more). Traces cables behind walls, up to 3-5 feet away in distance mode. Digital tone is a lot more powerful than the traditional "Fox and Hound" used in telephone systems. The big advantage of the higher power is that a dead coax can be followed until the signal stops if a cut is suspected. Cat5 Cable gets the signal on multiple pairs, so it will not show a single broken pair, but a cut wire is obvious. The sender unit also detects if there is a phone line, 2 phone lines, or an Active Ethernet network when it is plugged into a jack. Toner WILL Work on active Ethernet cables (Fox and Hound won't). Video (Click on the sub-videos in virtual demo, they compare Digital toner with Analog Fox and Hound)

Here is a cheaper one That is digital, doesn't trace. It works excellent for labeling RJ45 or Coax jacks that weren't labeled when installed, but doesn't allow "following" the path of a wire behind a wall, both ends must be accessible and un-broken. Video from Fluke demonstrating these

Analog Fox and Hound Designed for telephone systems, can be used on inactive networks, possibly coax if a DIY RJ11->Coax connector is made, but function wouldn't be anywhere near what the digital toner provides, Fox (probe) needs to be in contact with the wire, rather than near it.